The present invention relates to a method and apparatus for sampling a plurality of gases and analyzing or otherwise treating the sampled gases, and more particularly to a flow-chopping method and apparatus wherein a plurality of gases are collectively and sequentially sampled in desired proportions for gas analysis or other purposes.
For quantitative dilution, mixing or replacement of gases or for switching between zero reference gas and sample gas, a multi-channel valve is usually used. The individual gases supplied to the multi-channel valve from their gas supply sources through respective flow control valves are guided through the outlet passage of said valve to a take-out channel extended therefrom when the corresponding supply channels are opened by the mechanical channel-switching of said valve. With such channel-switching system, however, a transient pressure change occurs upon opening and closing of the channels in the switching valve, such pressure change, coupled with the complexity of individual adjustment of the gas supply channels, making it difficult to achieve the correct control of the amount of each gas. Further, in gas analysis the repeated opening and closing of channels for individual gases results in said gases being adsorbed at some portion of the flow system and each such adsorbed gas being removed by other gases, so that the base and signal values corresponding to zero reference gas or span gas and sample gases respectively become incorrect, and a substantial amount of time is expended before they are stabilized. On the other hand, in a gas analysis which does not need the switching of channels, although there are no problems caused by the above-mentioned transient pressure change or adsorption of gas by the flow system, inherent problems due to the use of a plurality of channels for sample and reference gases do arise.
Thus, a construction in which a detector consists of a sample cell and a reference cell, for example, a pair of heat conductivity cells or a non-dispersed infrared analyzer is supplied or charged with a compensation gas or reference gas for setting in the reference cell a zero level reference for signal values, and the detector produces as its output a signal which corresponds to the value of sample gas minus the value of reference gas, so that relatively accurate quantitative analysis can be carried out. However, the accuracy and the stability of sample and reference cells are fixed by the inherent characteristics of these cells and by the arrangement of gas channels and of the optical system, and intrinsic errors resulting therefrom cannot be eliminated.
In other detection systems, such as a flame ionization-detector and a chemiluminescence detector for deriving a signal only concerning one gas, if an arrangement intended to simultaneously detect at least two gases is employed the above drawback should become more noticeable. More specifically, in such cases, a photochemical or physical excitation-reactor must be provided for each of the gases, and it is very difficult to equalize the conditions of these reactors.
We have investigated the various drawbacks in the gas analyzing system described above, which handle a plurality of gases, and we have come to the conclusion that if different gases one of which may be a zero reference gas are successively taken-out of the streams of the gases little by little and fed as a series gas stream into a detection-cell, excitation-chamber or other chamber, then the foregoing absorption and decomposition problems about a multi-channel valve will disappear and a detected value for each gas which repeatedly appears can be obtained at the detector, contributing much to the stablization of gas analysis and the collective measurement of different samples. If such gas analyzing system is used, it is also clear that a two-cell system for sample and reference gases may be replaced by a single cell to compare two gases therein, so that the error due to cell conditions can be eliminated.